In some wireless local area networks (WLANs) such as wireless docking networks, one or more stations may be mobile devices typically powered by battery and another device may be a dock typically AC powered. Areas that have multiple devices and docks and/or multiple devices that connect with the same dock, may be referred to as dense environments from the perspective of the wireless communications and the greater the density, the more likely that a collision of transmissions from different stations may occur.
In order to avoid collisions, stations may implement a listen before talk, time-sharing mechanism also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). CSMA/CA assumes that any station's clear channel assessment mechanism has full information of the channel that includes physical and virtual (Network Allocation Vector) carrier sense. The physical carrier sense typically involves listening to or sampling a channel to detect a transmission prior to initiating a transmission. The virtual carrier sense may invoke the Network Allocation Vector (NAV) in transmissions to inform stations of a remaining duration of an ongoing transmission, which allows the non-participating stations to save power by going to sleep during the remainder of the transmission.
There are substantial limitations to the assumption of full information of the channel due to power limitation of the battery-powered device and due to interferences from hidden nodes. Hidden nodes may be nodes whose transmissions are hidden from the physical carrier sense, that are transmitting outside the receiving range of another station due to actual distance or obstacles, and that transmit interfering communications. Or hidden nodes may be nodes whose transmissions are hidden from the virtual carrier sense in that they are not transmitting while a station is listening to the channel but may transmit interfering communications. Mobile stations that need to operate with low power consumption, such as battery powered devices, may exacerbate this hidden node problem by entering a sleep mode while not actively transmitting.
Directive transmissions that may be implemented in, e.g., high speed transmissions such as transmissions in the 60 GHz (gigahertz) band, are particularly susceptible to the hidden node problem due the lowered ability to transmit around obstacles and the purposeful reduction in transmission distances, other than the direction of the receiving station, through constructive and destructive waveforms in a process referred to as beamforming.
Strong interference from a hidden node may prevent the station from receiving communications. For instance, a receiving station receives every signal directed at the receiving station but can only demodulate and decode a single signal transmission. If a hidden node begins transmitting to a receiving station while another station is also transmitting to that station, the receiving station may have so much interference that it may not receive either signal.